Semiconductor gas sensor, gas sensor system and method of gas analysis

ABSTRACT

A semiconducting gas sensor includes a gas-sensitive layer, a heater for heating the layer to a defined measuring temperature, and contact electrodes for measuring the electrical resistance of the gas-sensitive layer enclosed within a microchamber, in which the gas-sensitive layer is arranged. The chamber can be sealed from the outside, and is constructed so that the chamber volume is small enough to allow at least one component of the gas or gas mixture that is to be analyzed to be at least largely exhausted via conversion on the gas-sensitive layer, within a predetermined measuring interval. With the limited gas store and the conversion of a component of the gas during the measurement process, gases or gas mixtures comprising several components can be analyzed. In this, the measuring signal is reexamined following the conversion of at least one component. Within the chamber, several sensor elements may be arranged with gas-sensitive layers, and may be operated at different temperatures. One gas sensor system, for example, is comprised of at least two semiconducting gas sensors having microchambers, which are arranged within a system of gas lines and valves, and can be filled individually.

This application claims the priority of PCT International ApplicationNo. PCT/DE00/01510, filed 12 May 2000 and German patent document No. 19924 906.7 filed 31 May 1999, the disclosure of which is expresslyincorporated by reference herein.

The present invention relates to a semiconducting gas sensor inaccordance with the preamble of Patent claim 1, a gas sensor system, anda method of gas analysis using a semiconducting gas sensor.

The present invention relates to a semiconducting gas sensor, and to amethod of gas analysis using a semiconducting gas sensor,

In some fields, gas analysis is of great importance. For example, in thecombustion of fossil fuels, carbon monoxide and nitrous oxides orNO_(x), are produced, which may then be converted to O₃. The damage tothe environment caused by these substances is considerable. For thisreason it is highly imperative that exhaust gases produced by internalcombustion engines be analyzed, with an eye to reducing their emissionof pollutants.

One possibility for gas analysis is presented by semiconducting gassensors, in which a gas-sensitive metallic oxide layer, such as SnO₂, isbrought to a specific measuring temperature. By measuring the electricalresistance of the gas-sensitive layer at a specific temperature, the gasconcentrations, for example of CO, NO_(x), or O₃, can be determined.

The article by B. Ruhland, et al., “Gas-Kinetic Interactions of NitrousOxides with SnO₂ Surfaces”, Sensors and Actuators B 50 (1998) pp 85-94,discusses a semiconducting gas sensor of this type. In this known gassensor, a thin layer of SnO₂ is placed on a heating structure. An SiO₂layer separates a heating element from the gas-sensitive SnO₂ layer. Theheating structure with the gas-sensitive layer is arranged on a Si₃N₄membrane, which is then placed over a silicon substrate. In themeasurement process, the gas that is to be analyzed flows over thesensor element. The bombardment with the gas components to be analyzedcan also be accomplished via diffusion.

In the measurement of gases comprising several components, the problemarises that the effects of the individual gas components may becomesuperimposed in the measuring signal. For example, at a measuringtemperature of 400° C., a bombardment of the gas-sensitive layer with COor NO leads to a reduction in the electrical resistance of thegas-sensitive layer, while a bombardment with NO₂ at this temperatureresults in an increase in electrical resistance. Furthermore, thecontact between the gas-sensitive layer and ozone results in increasedresistance. For this reason, the individual concentrations in the gasmixture frequently cannot be precisely determined.

One possibility for solving this problem consists in providing anarrangement comprising several sensors having different measuringtemperatures. While a considerable degree of NO₂ sensitivity is presenteven at relatively low temperatures of 150° C. to 250° C., a suitablemeasuring temperature for CO, for example, lies between 350° C. and 450°C. The arrangement with the whole sensor array, however, is expensive,and thus associated with relatively high costs.

Another approach to solving the problem involves obtaining comparativesets of data for defined individual gases and gas mixtures at varioustemperatures via experimentation. To this end, the above-mentionedpublication provides for a bombardment of several sensor elements withindividual gas components at defined concentrations, in order todetermine the behavior of electrical resistance, as a function oftemperature. With the resistance behavior determined in this manner, itis then possible to analyze a gas mixture comprised, for example, of COand NO₂ using two sensors, wherein one sensor is operated at 200° C. andone sensor is operated at 400° C. One disadvantage of this process isthat it allows only very simple gas mixtures to be analyzed.Furthermore, interactions between the gases are not taken into account.

In addition, the high O₃ sensitivity disrupts the measurement processsignificantly. In many cases, the ozone sensitivity outweighs all othereffects. For example, with ozone concentrations that are higher than 100ppb the measuring signal can be interpreted only as an ozone signal.

It is thus one object of the present invention to create asemiconducting gas sensor and a gas sensor arrangement that is suitablefor analyzing a gas or gas mixture comprising a number of components,such as, for example, ozone and that can be produced simply andcost-effectively. Furthermore, a method of gas analysis is to beprovided, which will enable the analysis of a gas or gas mixturecomprising a number of components via semiconducting sensors.

This and other objects and advantages are achieved by the semiconductinggas sensor according to the invention, which comprises a gas-sensitivelayer, whose electrical conductivity can be altered via contact with agas, a heating apparatus for heating the layer to a defined measuringtemperature, contact electrodes for measuring the electrical resistanceor the electrical conductivity of the gas-sensitive layer, and a chamberin which the gas sensitive layer is positioned. The chamber can besealed from the outside; and the volume of the chamber is small enoughthat at least one component of the gas or gas mixture is largelyexhausted via conversion, within a predetermined measuring interval, forexample on the gas-sensitive layer.

In this manner, the disruptive effects of ozone on the measurementprocess can be eliminated.

With the small chamber volume, individual components of the gas becomeconverted during the measuring process, so that they do not contribute,or contribute only slightly, to the measuring signal. The remainingmeasuring signal is then no longer superimposed by the effects of thegas components that have already been converted, allowing theconcentrations of the remaining components to be more easily determined.With the invention it is possible to determine the concentrations ofdifferent gas components in a gas mixture, without requiring a multitudeof sensors operating at different temperatures, which require costlyevaluation. In addition, the gas analysis can be accomplished within arelatively short period of time, with the chamber volume being dependentupon the type of gas to betanalyzed and the desired duration of themeasuring interval.

Advantageously, the semiconducting gas sensor comprises a regulatingdevice that enables the heating of the gas-sensitive layer in stages,thus allowing individual components of the gas mixture to be selectivelyconverted at predetermined measuring temperatures. Preferably, thesemiconducting gas sensor is produced using micromechanical technology,for example via Si technology. This enables a simple, cost-effectiveproduction, and a standard implementation of the sensor.

A platinum heating resistor, arranged in a meandering pattern, ispreferably used as the heating device. The contact electrodes arepreferably also made of platinum. This serves to produce increasedtemperature stability, while preventing mutual interference between theelectrodes and the resistance material.

Advantageously, a passivating layer, comprised, for example, of SiO₂, ispositioned between the heater and the gas-sensitive layer and serves asan insulator. Specifically, a silicon substrate may be provided as thesupporting material, along with a nitride membrane, which separates theheater from the substrate.

The gas-sensitive layer is preferably comprised of SnO₂, however it canalso be made of other metallic oxides such as WO₃ and titanium oxide, orof organic materials such as phthallocyanine.

The semiconducting gas sensor is preferably designed to be suitable formeasuring concentrations of CO, NO₂, NO, and/or O₃. The chamber ispreferably a microchamber made, for example, of silicon. The chambervolume advantageously measures approx. 10 to 500 μl, preferably 10 to100 μl, and most preferably approx. 40 μl.

In accordance with a further aspect of the invention, a gas sensorsystem is provided, which comprises several semiconducting gas sensorsas specified in the invention, along with an arrangement of regulatedvalves and lines for the inlet and outlet of gas. In this manner, it ispossible to create redundancies, and to cost-effectively increase thelifespan of the system. In addition, a multitude of gas sensors may beused individually, allowing an improvement in measuring quality orprecision to be achieved. Preferably, the semiconducting gas sensors arearranged in a parallel connection, wherein the valves may be controlledindividually.

In accordance with a further aspect of the invention, a method of gasanalysis using a semiconducting gas sensor is provided, which comprisesthe following steps: preparing a semiconducting gas sensor with agas-sensitive layer in a sealable chamber; filling the chamber with agas or gas mixture that is to be analyzed, and sealing the chamber;heating the gas-sensitive layer to a predetermined measuringtemperature, and examining a measuring signal that is dependent, forexample, upon the electrical conductivity or the ohmic resistance of thegas-sensitive layer, at a moment of measurement in which at least onegas component has been exhausted via conversion within the chamber sothat it does not contribute, or contributes relatively slightly, to themeasuring signal; and determining of the content of at least oneremaining gas component from the remaining measuring signal.

With this process, gases or gas mixtures comprising several components,including ozone, may be easily analyzed, while the cost remains low, andthe evaluation process implemented can be simple.

Advantageously, the measuring signal is used at least two differenttimes during the measurement process to determine the components of thegas. For example, from the peak of the measuring signal and itssubsequent decrease, the concentration of at least one component of thegas can be determined. Advantageously, the measuring temperature lieswithin a range of approx. 20° C. to 550° C., preferably within a rangeof approx. 50° C. to 400° C., and most preferably within a range ofapprox. 200° C. to 400° C. Therein, the heating process may beimplemented gradually or in stages, with measurements being taken atdifferent temperature stages. Most preferably, a semiconducting gassensor as specified in the invention and/or a gas sensor system asspecified in the invention are used in this process.

The invention will be described below by way of example using theattached figures, wherein

FIG. 1 shows a schematic illustration of a cross-section of asemiconducting gas sensor as specified in the invention;

FIGS. 2a and b depict the reaction mechanisms for CO and NO on thegas-sensitive layer at 400° C.;

FIGS. 3a, b, and c depict various reaction mechanisms for NO₂ at atemperature of 200° C. or 400° C.;

FIG. 4 depicts the sensor signal for an ozone measurement;

FIG. 5 depicts the sensor signal for an NO₂ measurement;

FIG. 6 depicts the sensor signal for both a pure NO₂ measurement, and ameasurement of O₃, also in a gas mixture of NO₂ and O₃;

FIG. 7 depicts the sensor sensitivity as a function of theconcentrations of various gases;

FIG. 8 depicts the measuring signal for a gas in which nitrous oxide andO₃ molecules are present;

FIG. 9 shows an illustration in principle of the gas sensor systemspecified in the invention; and

FIG. 10 depicts the conversion rate of ozone under no-flow conditions asa function of various ozone concentrations.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a gas sensor 10 as specified in the invention. On a waferor silicon substrate 1, which serves as the carrier, a nitride membrane2 is positioned as a passivating layer. The nitride membrane 2 serves asan etch stop in the production of the wafer. On top of the membrane 2 isa heater 3 formed by a platinum heating resistor. The platinum heatingresistor is arranged in a meandering pattern on top of the membrane 2,and is operated at a voltage of up to approx. 5 volts. The platinumheating resistor or heater 3 possesses a high level of temperaturestability, hence the amount of sensor drift experienced is quite low. Ontop of the heater 3, a passivating layer 4 is positioned, which servesas an insulator and is made of SiO₂. This provides the advantage of asimple and precise method of production, which is also cost-effective.On top of the passivating layer 4 is the gas-sensitive layer 5 made ofSnO₂. Contact electrodes 6 a and 6 b are also arranged on thepassivating layer 4, for the purpose of determining the electricalresistance of the gas-sensitive layer 5. The sensor element constructedin this manner is positioned within a chamber 7, which can be sealedfrom the outside via valves 8 a and 8 b. The chamber 7 is small enoughthat gas components are converted within the chamber during themeasuring process, and, after a predetermined measuring interval, nolonger contribute to the measuring signal, or contribute onlyinsignificantly.

In the present case, the chamber 7 is designed as a microchamber,wherein the chamber volume is approx. 0.5 cm³. Depending upon thepurpose of the measurement or upon the gas to be analyzed, substantiallysmaller chamber volumes are also possible, for example within the rangeof approx. 0.05 cm³. Chambers this small can be produced veryeffectively using microtechnology. During the measuring process,so-called “no flow” conditions prevail within the chamber, in otherwords only a limited store of individual gases is present, which are atleast partially converted. Via the valves 8 a, b and gas lines 9, thechamber 7 can be filled, and sealed from the outside when measurementbegins. This serves to prevent any subsequent diffusion of gascomponents, such as may occur, for example, with macroscopic gasvolumes. The chamber 7 may also possess a larger volume of up to a few100 μl. However, a volume that is between 10 and 100 μl is especiallyadvantageous. In the preferred embodiment, the chamber is made ofsilicon.

Since the volume of the chamber is dependent upon the specific measuringpurpose, the size of the chamber 7 must be designed and constructedaccordingly. For example, the volume of the chamber 7 must be designedsuch that the gas will be allowed to diffuse over a predeterminedmeasuring interval from the most remote point of the chamber to thesensor element or to the gas-sensitive layer 5. The conversion of atleast one gas component then follows, so that the store of gas becomesexhausted or converted in terms of this at least one component, withinthe desired measuring interval. It is not necessary, however, for acomplete conversion to take place; rather, it is sufficient for thesecomponents to no longer contribute significantly to the measuring signalat a specific point in time, or for these components to no longerdisrupt the signal. In other words, the conversion must proceed for aslong during the predetermined measuring interval that the concentrationsof the remaining components can be precisely determined.

In the embodiment illustrated here, the gas-sensitive layer 5 iscomprised of stannic oxide pellets, in other words, a polycrystallinesemiconducting material is present. Between the stannic oxide pelletsare potential barriers, which are modulated via bombardment with gases.For example, when the surface of the stannic oxide pellets is bombardedwith air, a surface coating with oxygen is produced, which istemperature dependent. The physisorbed or chemisorbed oxygen causes adepletion surface layer to form on the stannic oxide pellets atincreasing temperatures, in other words a potential barrier is formedbetween the individual crystallites. This causes the level of electricalconductivity to fall.

In FIG. 2a, the reaction mechanism for CO on the gas-sensitive layer 5is shown, based upon current knowledge. In this, a CO molecule isadsorbed on the surface, where it reacts with a surface oxygen ion,forming a CO₂ molecule and an electron:

CO_(gas)+O_(surface−)→CO_(2gas) +e ⁻

The CO₂ that is formed desorbs from the SnO₂ surface, and the electronis donated to the SnO₂. This causes an increase in the conductivity ofthe SnO₂ layer, that is, its electrical resistance drops. In otherwords, the depletion surface layer becomes smaller and the potentialbarrier is reduced. In this measurement process, a temperature of around400° C. is advantageous, since the sensitivity level is relatively-high.

In FIG. 2b, the reaction mechanism for an NO molecule is depicted as anexample. Obviously, the reaction mechanism for NO on the SnO₂ layer isvery similar to the reaction mechanism for CO. With an oxygen ion O⁻ onthe surface of the gas-sensitive layer, an NO molecule is adsorbed orchemisorbed, producing the following reaction:

NO_(gas)+O⁻ _(surface)→NO_(2gas) +e ⁻

NO and O₂ molecules are desorbed on the surface, and the electricalconductivity is increased. In this measurement process as well, thepreferred temperature range is between 400° C. and 500° C.

In the case of NO₂, a substantially more complex reaction process takesplace. FIG. 3a shows the reaction mechanism for NO₂ on the gas-sensitivelayer 5, at a temperature of up to 200° C. In this, an NO₂ molecule isadsorbed on the surface of the SnO₂ layer 5, and the following binds tothe surface, gaining an electron:

NO_(2gas) +e ⁻→NO⁻ _(2surface)

This causes the potential barriers on the SnO₂ grain boundaries to beincreased, and the electrical conductivity to decrease. An NO moleculeis then desorbed from the surface, and an O⁻ is returned to the surface:

NO⁻ _(2surface)→NO_(gas)+O⁻ _(surface)

At a higher temperature of ca. 400° C., a concentration-dependentreaction takes place. The reaction mechanism for NO₂ at a temperature of400° C. and at lower concentrations is depicted in FIG. 3b. The NO₂molecule binds to the O⁻ that is now present on the surface, after whichNO and O₂ molecules are desorbed. In this, an electron is donated to theSnO₂ layer, causing the electrical conductivity to increase:

NO_(2gas)+O⁻ _(surface)→NO_(gas)+O_(2gas) +e ⁻

At higher concentrations, however, the reaction is different. FIG. 3cdepicts the reaction mechanism for NO₂ on the SnO₂ surface at higherconcentrations and at a temperature of 400° C. In this, NO₂ moleculesare adsorbed, causing an electron to be acquired. NO then desorbs fromthe surface, and an O⁻ is returned to the surface. The electricalconductivity is reduced:

NO_(2gas) +e ⁻→NO_(gas)+O⁻ _(surface)

Furthermore, O₃ produces a strong effect on the measurement process,which influences the measuring signal substantially. Therein, an O⁻ isbound to the surface of the gas-sensitive layer 5, and an O₂ molecule isdesorbed from the surface. This reaction occurs primarily along theedges of the gas-sensitive layer 5, or in the case of thin layers. Withthicker layers, however, the deeper areas remain unaffected by O₃. Incontrast, other molecules, such as CO, penetrate to deeper areas wherethey react with the gas-sensitive layer 5 or with the SnO₂.

With the microchamber 7, the gas store within the chamber is limited.Via diffusion or flow, the O₃ molecules present in the chamber 7eventually reach the surface of the gas-sensitive layer 5, where theyare converted to O₂.

Of particular importance here is that residual O₂ molecules will notdisrupt the measurement, since the sensor element with the gas-sensitivelayer 5 will not react to the few O₂ molecules present.

Below, measurements conducted on various gases will be described.

FIG. 4 depicts the sensor signal as a function of time in an ozonemeasurement. First, the valves 8 a, 8 b of the chamber 7 (FIG. 1) areopened, in order to achieve a constant flow of gas within the chamber.This allows the sensor to be conditioned. The gas flow is comprised ofsynthetic air with 30% relative humidity. This state is illustrated inFIG. 4 by the area 1. Therein, the sensor is at equilibrium with thesurrounding, flowing air. The valves 8 a, 8 b are then closed, causingthe humidity within the chamber to rise as a result of the heating ofthe chamber, as water desorbs from the chamber walls. The sensor signaldrops and the final value is stored in a memory device as a calibrationor zero gas value (area 2). The humidity reduces the electricalresistance of the SnO₂ layer, since the water on the SnO₂ layer formsOH⁻ groups, wherein atomic hydrogen is released, resulting in areduction of the SnO₂ layer.

In area 3 of FIG. 4, the valves 8 a, 8 b are reopened, causing thesensor to be reconditioned in the gas flow. The air humidity drops, andthe measuring signal rises. In this area, the original conditions arereproduced. The area 4 depicts the addition of ozone (50 ppb) to the gasflow.

Now, the valves 8 a, 8 b are again closed (area 5). The actualmeasurement is conducted. In the chamber 7 a no-flow situation prevails,and the humidity within the chamber increases due to the heating of thechamber, as a desorption of H₂O from the chamber walls takes place. Thesensor signal drops, and the ozone in the chamber breaks down to O₂.This slight increase in O₂ concentration is not recognized by thesensor. Thus the sensor signal S in the area 5 achieves the same finalvalue as in area 2. Then, in area 6, the valves are reopened, theoriginal conditions are reproduced. What is decisive in this measurementprocess is that the sensor no longer recognizes the O₃ after a certainperiod of time, which is a function of the chamber volume and theconcentration of O₃. Hence, the measuring signal is no longer affectedby the O₃ after a certain period of time.

FIG. 5 depicts an NO₂ measurement. In comparison with an O₃ measurement,which is depicted in FIG. 4, here the sensor signal does not drop backdown to the original value in area 5. Here, NO₂ desorbs as NO and O₂from the stannic oxide surface, which is oxidized to a higher valency inthe sealed measuring chamber 7, back to NO₂, since the chemicalequilibrium at near room temperature is NO₂ or N₂O₄. The magnitude ofthe offset in area 5 is a measurement of the NO₂ concentration in thesealed measurement chamber 7.

The ozone concentration or the NO₂ concentration is determined byadjusting a function to fit the measuring signal, and comparing thiswith values in a reference table. The measuring signal follows an efunction of the form S=A+B×EXP (t/tau). If this function is adjusted tofit the measuring signal, then the end value that will be establishedafter an infinitely long period of time is obtained. The signal valueobtained in this manner is assigned to an ozone concentration using areference table.

With the NO₂ measurement in FIG. 5, the reference value from area 2 isderived from the calculated final value from area 5. This value is thenalso assigned an NO₂ concentration using a reference table.

FIG. 6 depicts the measuring signal for both a pure NO₂ measurement andan O₃ measurement, as well as a measurement conducted using a gasmixture comprised of NO₂ and O₃. Herein, it is clear that with thecombined addition of NO₂ and O₃ the sensor signal follows the course ofthe NO₂ measurement. Hence it follows that the percentage of NO₂ can bedetermined from the mixed signal in the no-flow situation. From thecomposite signal in the flow situation, with a low NO₂ concentration,the ozone concentration can be deduced directly, since the response ofthe sensor to ozone is much greater than its response to NO₂. Withhigher NO₂ concentrations, the ozone concentration is calculated usingthe measured NO₂ concentration.

FIG. 7 depicts the sensor sensitivity as a function of theconcentrations of various gases. Herein, it is clear that thesensitivity to ozone outweighs the sensitivity to all other gases. Forsignal evaluation this means that two cases must be differentiated. Withozone concentrations that are greater than 100 ppb, the measuring signalcan be interpreted directly as an ozone signal. However, forconcentrations that are below 100 ppb, the ozone signal must becorrected by the NO₂ component, as this gas produces the next highesteffect on the sensor. The correction is made using the NO₂ value that isobtained via the no-flow measurement.

Gases like CO and CH₄ can be completely converted to CO₂ and H₂O.Therein, CO₂ produces no effect on the measuring signal, and theresulting quantity of H₂O at low CH₄ concentrations is too small toproduce an effect on the measuring signal. If, during the measurementprocess, gases should appear that have not been tested, but that coulddistort the measurement results, suitable filters are selected, such asare customarily used in gas-sensor technology, and are known to expertsin the field.

The example illustrated in FIG. 8 shows a measurement of a gas in whichnitrous oxide and O₃ molecules are present. By opening the valves 8 a, 8b, the measuring chamber 7 is filled via the gas lines 9 with the gasthat is to be analyzed.

With the heater 3, the gas-sensitive layer 5 is brought to apredetermined measuring temperature, or is held at this temperature ifit has already been heated. The measuring temperature in this case is400° C. The electrical resistance of the gas-sensitive layer 5 ismeasured via the contact electrodes 6 a, 6 b.

FIG. 8 shows the measuring signal S as a function of time t. At thestart of the measurement, the signal S remains nearly constant, butincreases sharply at time t1, and reaches a maximum level at time t2.This is followed by a relatively rapid drop in the signal S, and at timet3 it reaches a nearly constant value. At the start of the measurement,the measuring signal is determined mainly by the amount of O₃ present,in other words the effects of the nitrous oxide are completely masked.The sharp increase of the signal S up to time t2 is based upon theabove-described reaction of the O₃ molecules on the SnO₂ surface.However, because the store of O₃ molecules in the chamber 7 is limited,and the O₃ molecules are eventually converted, a reduction of themeasuring signal or the ohmic resistance in the SnO₂ layer now occurs,that is, after time t2. At time t3, the signal is then completely oralmost entirely unaffected by O₃, and the signal value S at time t3 ischaracterized substantially by the concentration of the nitrous oxide orNO that is present. From this, the concentration of the nitrous oxide inthe gas mixture can be determined, without influence by O₃.

Depending upon the purpose of the measurement or upon the establishedrequirements, several sensor elements may also be arranged withgas-sensitive layers 5 in the chamber 7. These can, for example, beoperated at different measuring temperatures, so that in theabove-described manner the measurement of different gas components attime t3 can be determined. It is also possible to determine theconcentration of different gas components via comparison with measuredvalues at known concentrations. Herein, the measured value is applied ata time in which the O₃ components, or at least one component, has beencompletely or nearly completely converted, so that the remainingmeasuring signal characterizes the concentrations of the remainingcomponents.

FIG. 9 depicts a preferred embodiment of a gas sensor system inaccordance with the present invention. Herein, two semiconducting gassensors 10 a, 10 b are arranged parallel to one another within a systemof gas lines 90. The semiconducting gas sensors 10 a, 10 b weredescribed above. Via regulated valves 80, the measuring chambers 70 a,70 b of the semiconducting gas sensors 10 a, 10 b may be filled. Thechambers 70 a, 70 b can be filled with gas and sealed individually basedupon requirements or upon the intended measurement.

FIG. 10 depicts the conversion rate of ozone under no-flow conditions asa function of various ozone concentrations. Recorded here is thereaction rate, expressed via the gradient of the measuring curve,against various ozone concentrations. In a large measuring chamber, noconversion of ozone takes place within a reasonable interval of time. Asmall chamber size results in ozone conversion, and a further reductionof the volume increases the conversion rate.

In order to minimize the dead volume within the chamber 7 or thechambers 70 a, 70 b, flange-type valves are used. The gas flow system inthe preferred embodiments is made of polyether etherketone (PEEK). Thismaterial is especially resistant to the gases described here. The gasflow system is implemented via specially designed bores in a plate or ina carrier material, which serves to further minimize the dead volume.The particular advantages of this design are the small chamber volumeand the possibility of a measurement conducted in a no-flow situationwithin a particularly small chamber volume. By producing the chamberfrom silicon using micromechanical technology, the volume can be madeextremely small, enabling simple methods of production and maintenance,and resulting in a considerable decrease in cost.

What is claimed is:
 1. Semiconducting gas sensor comprising: agas-sensitive layer whose electrical conductivity can be varied bycontact with a gas; a heater for heating the layer to a predefinedmeasuring temperature; contact electrodes for measuring the electricalresistance or the conductivity of the gas-sensitive layer; and a chamberin which the gas-sensitive layer is arranged; wherein, a valvearrangement is provided to seal the chamber from the outside and keep itclosed during the measurement process, whereby during measurement alimited supply of individual gases in the chamber is at least partiallyconverted; and volume of the chamber is such that at least one componentof a limited gas store within the chamber is substantially exhausted viaconversion within a predetermined measuring interval.
 2. Thesemiconducting gas sensor in accordance with claim 1, further comprisinga device for regulating heating of the gas-sensitive layer in stageswhereby individual components of the gas can be selectively converted atpredetermined measuring temperatures.
 3. The semiconducting gas sensorin accordance with claim 1, wherein the gas sensor is produced usingmicromechanical technology.
 4. The semiconducting gas sensor inaccordance with claim 1, wherein the heater is a platinum heatingresistor which is arranged in a meandering pattern.
 5. Thesemiconducting gas sensor in accordance with claim 1, further comprisinga passivating layer that is positioned between the heater and thegas-sensitive layer, and is made of SiO₂.
 6. The semiconducting gassensor in accordance with claim 1, wherein the contact electrodes aremade of platinum.
 7. The semiconducting gas sensor in accordance withclaim 1, further comprising a silicon substrate as a carrier, and anitride membrane, which separates the heater from the carrier.
 8. Thesemiconducting gas sensor in accordance with claim 1, wherein thegas-sensitive layer is made of a material selected from the groupconsisting of SnO₂, WO₃, titanium oxide, and organic materials.
 9. Thesemiconducting gas sensor in accordance with claim 8, wherein theorganic materials comprise phthallocyanine.
 10. The semiconducting gassensor in accordance with claim 1, wherein the gas sensor is configuredfor measuring concentrations of CO, NO₂, NO, and/or O₃.
 11. Thesemiconducting gas sensor in accordance with claim 1, wherein thechamber is made of silicon.
 12. The semiconducting gas sensor inaccordance with claim 1, wherein the chamber volume measures no morethan 0.05 to 10 cm³.
 13. The semiconducting gas sensor in accordancewith claim 1, wherein the chamber volume measures no more than 0.03 to0.7 cm³.
 14. The semiconducting gas sensor in accordance with claim 1,wherein the chamber volume measures no more than 0.5 cm³.
 15. A gassensor system comprising: a plurality of semiconducting gas sensors inaccordance with claim 1; and lines for the inlet and outlet of gas via avalve arrangement of controllable valves.
 16. The gas sensor system inaccordance with claim 15, wherein the semiconducting gas sensors arearranged in a parallel connection.
 17. The gas sensor system inaccordance with claim 15, wherein the valves are controllableindividually.
 18. A method of gas analysis using a semiconducting gassensor, comprising: providing a semiconducting gas sensor with agas-sensitive layer; placing said gas sensor in contact with a gas orgas mixture that is to be analyzed; heating the gas-sensitive layer; andanalyzing a measuring signal that is a function of the electricalconductivity of the gas-sensitive layer; wherein, the semiconducting gassensor is provided in a sealable chamber; the chamber is filled with thegas or gas mixture that is to be analyzed, and is sealed; thegas-sensitive layer is held at a predetermined measuring temperature;the measuring signal is examined when at lest one component of the gashas been exhausted via conversion within the chamber, to a point atwhich it no longer supplies any significant contribution to themeasuring signal; and a content of at least one remaining gas componentis determined from the remaining measuring signal.
 19. The method inaccordance with claim 18, wherein: the measuring signal is examined as afunction of time; and the measurement is used at least two differenttimes to determine the gas components.
 20. The method in accordance withclaim 18, wherein the concentration of at least one gas component isdetermined from a maximum peak of the measuring signal and a subsequentdrop thereof.
 21. The method in accordance with claim 18, wherein themeasuring temperature lies within a range of 20° C. to 550° C.
 22. Themethod in accordance with claim 18, wherein the measuring temperaturelies within a range of 50° C. to 400° C.
 23. The method in accordancewith claim 18, wherein the measuring temperature lies within a range ofapproximately 200° C. and 400° C.
 24. The method in accordance withclaim 18, wherein: heating is gradual; and measurements are taken atdifferent measuring temperatures.
 25. The method in accordance withclaim 18, wherein said gas sensor comprising a gas sensor according toclaim
 1. 26. The method in accordance with claim 18, wherein said gassensor comprises a gas sensor system according to claim 16.